1. Field of the Invention
The present invention relates to a vehicular steering system for steering rearward wheels as well as forward wheels, and more particularly to a steering system for steering rearward wheels of a motor vehicle under electronic control.
2. Description of the Prior Art
Conventional motor vehicles include a forward wheel 30, as shown in FIG. 7 of the accompanying drawings, which is dirigible by the rotation of a steering wheel. When the steering wheel starts being rotated, the forward wheel 30 is turned to form a slip angle .alpha..sub.1 with respect to the longitudinal axis of the vehicle and to produce a lateral force F.sub.1, thus initiating turning movement of the vehicle. At this time, there is generated a lateral acceleration a, or a yaw rate (yaw angular velocity) produced by a yawing motion of the vehicle about a vertical axis at the center of gravity of the vehicle. Then, the vehicle starts turning, whereupon a rearward wheel 31 forms a slip angle .alpha..sub.2 with respect to the longitudinal axis of the vehicle and is subjected to a lateral force F.sub.2 (FIG. 8), increasing the lateral acceleration a. With such a motor vehicle wherein only the forward wheel is dirigible, generation of the lateral force at the rearward wheel is delayed from that of the lateral force at the forward wheel, thus resulting in a time delay before the rearward wheel 31 is involved in the turning motion of the vehicle. Such a delay in the response of the rearward wheel becomes greater as the vehicle speed increases since the turning angle of the vehicle and the maximum slip angle .alpha..sub.2 of the rearward wheel 31 becomes larger with higher vehicle speeds at the same steering angle. Due to such a response delay of the rearward wheel which increases as the vehicle speed increases, there is a tendency for the prior vehicle to fail in quickly following the driver's steering action.
To cope with the response delay of the rearward wheel, there has been proposed a steering system for steering rearward wheels as well as forward wheels. With such a steering system installed on a motor vehicle, rearward wheels are mechanically interlinked to forward wheels for their steered motion. There is a need, however, to control the rearward wheels so as to meet driving conditions of the vehicle during a turning motion.
When a vehicle is accelerated or decelerated, turning characteristics thereof are different from those of the vehicle as it turns at a constant speed even if their speeds of travel are the same as a certain point of time. More specifically, as the vehicle is accelerated or decelerated, distribution of the vehicle load on forward and rearward wheels changes to thus cause forward and rearward suspensions to move upwardly and downwardly in strokes thereof, resulting in changes in over or under steering and camber at the forward and rearward wheels. Thus, when the vehicle is accelerated or decelerated while driving in a normal circular motion, the radius of turning motion of the vehicle changes. For example, when accelerated while being turned, a front-wheel-driven vehicle will turn along a curve having a larger radius of curvature than that of the curve followed by the vehicle turning at a constant speed, and conversely a rear-wheel-driven vehicle will turn along a curve having a smaller radius of curvature. Maneuverability and stability of vehicles having dirigible forward and rearward wheels can be improved by reducing changes in the turning characteristics which would be produced when the vehicle is accelerated and decelerated while in a turning motion. It is also desirable in such vehicles to select a steered angle ratio between forward and rearward wheels so as to allow the driver to have a better steering feeling.